Light diffusing film and process for producing the light diffusing film

ABSTRACT

A light diffusing film having superior productivity can be obtained by using very short fibers as a substitute for conventional spherical fine particles as light diffusing material. It is possible to mass-produce very short fibers at low cost, for example, by cutting fibers. Further, it is relatively easy to obtain very short fibers with a narrow fiber length distribution. The use of this makes it possible to carry out more highly sophisticated optical design of light diffusing films.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for producing a lightdiffusing film in which a plurality of very short fibers are dispersedin a film made of a translucent resin.

2. Description of Related Art

Light diffusing films are used for various displays for the purpose ofmaking light intensity distribution of light from a light source uniformand avoiding unevenness in brightness of screens. Conventionally, filmsin which spherical fine particles with small and large diameter aredispersed in each film made of a translucent resin as light diffusingmaterial are known as light diffusing films (Japanese Patent ApplicationLaid-open Publication No. JP 2003-43218 A). Such light diffusing filmsare capable of obtaining desired light diffusing characteristics byadjusting the refractive index or the size of the spherical fineparticles.

However, such conventional light diffusing films had disadvantages ofhigh cost and poor productivity because as the particle size ofspherical fine particles used for these light diffusing films becamesmaller, it became more difficult to mass-produce these films, resultingin high cost. Therefore, novel light diffusing films which solve suchproblems have been demanded.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a light diffusingfilm which is easy to mass-produce its material at low cost and hassuperior productivity, and a process for producing the light diffusingfilm.

It has revealed that as a result of studies of inventors of the presentinvention, a light diffusing film having superior productivity and aprocess for producing thereof can be obtained by using very shortfibers.

The summary of the present invention is as follows:

In a first preferred embodiment, a light diffusing film according to thepresent invention comprises: a film made of a translucent resin; and aplurality of very short fibers dispersed in the film made of atranslucent resin, wherein an average refractive index n_(A) of thetranslucent resin is different from an average refractive index n_(B) ofthe very short fibers when the average refractive index n_(A) of thetranslucent resin is defined as (extraordinary refractiveindex+2×ordinary refractive index)/3 and the average refractive indexn_(B) of the very short fibers is defined as (refractive index in thedirection of a major axis+2×refractive index in the direction of a minoraxis)/3. The major axis direction of the very short fibers is a fiberaxis direction and the minor axis direction of the very short fibers isa direction orthogonal to the fiber axis direction.

In a second preferred embodiment of the light diffusing film accordingto the present invention, the average refractive index n_(A) of thetranslucent resin is 1.3 to 1.7 and the average refractive index n_(B)of the very short fibers is 1.4 to 1.6, and an absolute value of thedifference between the average refractive index n_(A) of the translucentresin and the average refractive index n_(B) of the very short fibers,|n_(A)−n_(B)| is 0.005 to 0.15.

In a third preferred embodiment, the light diffusing film according tothe present invention comprises: a film made of a translucent resin; anda plurality of very short fibers dispersed in the film made of atranslucent resin, each of which has a first refractive index region anda second refractive index region provided within the first refractiveindex region, wherein an average refractive index n_(A) of thetranslucent resin is different from an average refractive index n_(B2)of the very short fibers in the second refractive index region when theaverage refractive index n_(B2) of the very short fibers in the secondrefractive index region is defined as (refractive index in the directionof a major axis+2×refractive index in the direction of a minor axis)/3.The major axis direction of the very short fibers in the secondrefractive index region is a direction of a fiber axis in the sameregion and the minor axis direction is a direction to be orthogonal tothe direction of the fiber axis.

In a fourth preferred embodiment of the light diffusing film accordingto the present invention, an average refractive index n_(A) of thetranslucent resin is 1.3 to 1.7 and an absolute value of the differencebetween the average refractive index n_(A) of the transparent resin andan average refractive index n_(B2) of the very short fibers in thesecond refractive index region, |n_(A)−n_(B2)| is 0.01 to 0.15.

In a fifth preferred embodiment of the light diffusing film according tothe present invention, an average refractive index n_(A) of thetranslucent resin, an average refractive index n_(B1) of the very shortfibers in the first refractive index region, and an average refractiveindex n_(B2) of the very short fibers in the second refractive indexregion satisfies the relationship: n_(A)<n_(B1)<n_(B2) orn_(B2)<n_(B1)<n_(A) when the average refractive index n_(B1) of the veryshort fibers in the first refractive index region is defined as(refractive index in the direction of a major axis+2×refractive index inthe direction of a minor axis)/3. The major axis direction of the veryshort fibers in the first refractive index region is a direction of thefiber axis in the same region and the minor axis direction of the veryshort fibers is a direction orthogonal to the direction of the fiberaxis.

In a sixth preferred embodiment, the light diffusing film according tothe present invention comprises: a film made of a translucent resin; aplurality of very short fibers dispersed in the film made of atranslucent resin; and a plurality of spherical fine particles dispersedin the film made of a translucent resin, wherein an average refractiveindex of the translucent resin is different from an average refractiveindex of the very short fibers and a refractive index of the sphericalfine particles. The average refractive index of the very short fibersand the refractive index of the spherical fine particles may beidentical or different.

In a seventh preferred embodiment, a process for producing theaforementioned light diffusing film according to the present inventioncomprises the steps of: A) dispersing a plurality of very short fibersobtained by cutting fibers in a liquid material which may form a filmmade of a translucent resin to obtain a dispersion liquid; and B)casting the dispersion liquid obtained in the step A in a film andsolidifying or curing a cast layer thereof to obtain a light diffusingfilm.

ADVANTAGE OF THE INVENTION

The present invention makes it possible to realize a light diffusingfilm having superior productivity and a process for producing the same.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As a result of a careful study conducted by the inventors of the presentinvention to resolve the above-mentioned problems, it has revealed thata light diffusing film having superior productivity can be obtained byusing very short fibers as a substitute for spherical fine particlesthat have been used for conventional light diffusing films as lightdiffusing material.

It is possible to mass-produce very short fibers to be used in thepresent invention at low cost, for example, by cutting fibers. Althoughit was conventionally difficult to obtain spherical fine particles witha narrow particle size distribution, it is relatively easy to obtainvery short fibers with a narrow fiber length distribution, for example,by appropriately adjusting the cutting width of the fibers. The use ofthis makes it possible to carry out more highly sophisticated opticaldesign of light diffusing films.

[Light Diffusing Film]

The light diffusing film to be used in the present invention comprises:a film made of a translucent resin; and a plurality of very short fibersdispersed in the film made of a translucent resin, wherein an averagerefractive index of the translucent resin is different from an averagerefractive index of the very short fibers. Very short fibers are usedbecause: (1) the very short fibers are convenient to achieve athree-dimensional random distribution of fiber orientation within thethin light diffusing film; and (2) the light diffusing efficiency of thelight diffusing film is superior because of having several end surfacesof the fibers. In this light diffusing film, it is possible toeffectively produce light diffusing films because it is possible tomass-produce very short fibers at low cost. Further, such a lightdiffusing film enables highly sophisticated optical design because it ispossible to minimize the fiber length distribution of the very shortfibers as narrow as possible.

The light diffusing film of the present invention can emit diffusionlight by refracting incident light at an interface between the veryshort fibers and the translucent resin. Since the light diffusing filmcan emit diffusion light, generally, the light diffusing film visuallylooks cloudy.

The very short fibers are preferably dispersed in such a state that thedistribution of orientation of the very short fibers (the distributionof orientation of a fiber axis of the very short fibers) is random inthree dimensions. However, the number of the very short fibers orientedin a direction perpendicular to the plane of the film may be relativelysmall as long as the orientation of the very short fibers is random inthe plane of the film. When the distribution of orientation of the veryshort fibers is closer to random in three dimensions, the more it ispossible to diffuse incident light in all directions all-around.

The degree of light diffusion of the light diffusing film of the presentinvention is determined by an absolute value of the difference betweenan average refractive index n_(A) of the translucent resin and anaverage refractive index n_(B) of the very short fibers, |n_(A)−n_(B)|.|n_(A)−n_(B)| is preferably 0.005 to 0.15, more preferably 0.01 to 0.10.

The haze value of the light diffusing film of the present invention isappropriately adjusted by adjusting the amount of the very short fibersto be added and typically has a haze of 10 to 90%. The amount of thevery short fibers to be added is preferably 10 to 50 wt %, morepreferably 15 to 40 wt %, with respect to the total weight of the lightdiffusing film.

The thickness of the light diffusing film of the present invention ispreferably 5 to 300 μm, more preferably 10 to 200 μm.

As shown in FIG. 1 (a), in one embodiment, a light diffusing film 10 ofthe present invention comprises: a film made of a translucent resin 12;and a plurality of very short fibers 11 dispersed in the film as lightdiffusing material. The light diffusing film 10 with such aconfiguration is at low cost and excellent in productivity.

As shown in FIG. 1 (b), in another embodiment, a light diffusing film 20of the present invention comprises: a film made of a translucent resin23; a plurality of spherical fine particles 21 dispersed in the film ofthe translucent resin 23 as light diffusing material; and a plurality ofvery short fibers 22 dispersed in the film made of the translucent resin23 as light diffusing material. An average refractive index of thetranslucent resin 23 (a portion including no very short fibers) isdifferent from an average refractive index of the very short fibers 22and an average refractive index of the spherical fine particles 21. Inthis case, the average refractive index of the very short fibers 22 maybe identical to or different from the refractive index of the sphericalfine particles 21. In the light diffusing film 20 of such aconfiguration, the very short fibers 22 are typically used instead ofspherical fine particles with a small particle size which was difficultto be used due to high cost. In this case, the diameter of the veryshort fibers 22 corresponds to the diameter of the spherical fineparticles with small particle size. Since the particle size distributionsubstantially has two peaks (the diameter of the very short fibers 22and the diameter of the spherical fine particles 21) because of thisconfiguration, it is possible to carry out more highly sophisticatedoptical design. In addition, the light diffusing film 20 is lessexpensive and has more superior productivity than the film usingspherical fine particles having small particle size.

[Very Short Fibers]

The very short fibers to be used in the present invention can betypically obtained by cutting fibers. In the present invention, the word“very short fiber” refers to one having a fiber length of 1 mm or less,and the word “fiber” refers to one having a fiber length larger than 1mm. The fiber length of the very short fibers to be used in the presentinvention is preferably 2 μm to 500 μm, more preferably 10 μm to 100 μm.

The cross-sectional shape of the very short fibers to be used in thepresent invention perpendicular to a fiber axis is not particularlylimited, and may be a circle, a polygon such as a triangle or aquadrangle, or a polygonal shape with rounded corners. The diameter ofthe very short fibers is preferably 2 μm to 50 μm, more preferably 2 μmto 30 μm. It is to be noted that when the cross-sectional shape of thevery short fibers is not a circle, the longest span between two pointsin their cross section is defined as a diameter.

The material of the very short fibers to be used in the presentinvention is not particularly limited, but a polymer material issuitable from the viewpoint of excellent workability, particularly, thepolymer material that is excellent in translucency and no colored ispreferable. Examples of such a polymer material include olefin-basedpolymers, vinyl alcohol-based polymers, (meth)acrylic-based polymers,ester-based polymers, styrene-based polymers, imide-based polymers,amide-based polymers, liquid-crystal polymers, and blended polymers oftwo or more of these polymers. Among them, olefin-based polymers, vinylalcohol-based polymers, and blended polymers of two or more of thesepolymers are preferably used.

The very short fibers to be used in the present invention may becomposed of one type of refractive index region or may be composed oftwo types of refractive index regions.

In the case where the very short fibers composed of one kind ofrefractive index region are used, the very short fibers preferably havean average refractive index n_(B) of 1.4 to 1.6. If necessary, theaverage refractive index n_(B)of the very short fibers can be increasedor decreased by changing the kind of organic group to be introduced intothe very short fibers and/or the amount of an organic group contained inthe very short fibers. For example, the refractive index of the veryshort fibers can be increased by introducing a cyclic aromatic group(e.g., a phenyl group) into the very short fibers. On the other hand,the refractive index of the very short fibers can be decreased byintroducing an aliphatic group (e.g., a methyl group) into the veryshort fibers.

Examples of the aforementioned very short fibers having two types ofrefractive index regions include so-called “core-sheath structured” veryshort fibers 30 with a second refractive index region 32 provided withina first refractive index region 31 shown in FIG. 2 (a) and very shortfibers 40 with two or more second refractive index regions 42 within thefirst refractive index regions 41 having a so-called island structureshown in FIG. 2 (b).

Although both the very short fiber 30 shown in FIG. 2( a) and the veryshort fiber 40 shown in FIG. 2( b) are composed of only the first andsecond refractive index regions, the very short fibers to be used in thepresent invention may have a third refractive index region (not shown)made of any material and/or an optically-isotropic region (not shown)made of any material. Further, the second refractive index region of thevery short fiber shown in FIG. 2( a) and the second refractive indexregions of the very short fiber shown in FIG. 2( b) are all cylindrical,but the shape of the second refractive index region is not particularlylimited, and may be a polygonal prism such as a triangular prism or aquadrangular prism or a polygonal prism with rounded corners. Further,the second refractive index regions do not always need to be evenlydistributed within the first refractive index region, and may beunevenly distributed within the first refractive index region.

In a case where the light diffusing film according to the presentinvention uses very short fibers each having a first refractive indexregion and a second refractive index region provided within the firstrefractive index region, the average refractive index n_(A) of thetranslucent resin, the average refractive index n_(B1) of the firstrefractive index region, and the average refractive index n_(B2) of thesecond refractive index region satisfy the relationship:n_(A)<n_(B1)<n_(B2) or n_(B2)<n_(B1)<n_(A). In the case of such a lightdiffusing film in which the average refractive index is changedstepwise, the difference in refractive index at an interface between twomembers is small, and therefore interfacial reflection occurring at theinterface between the translucent resin and the very short fibers can bereduced so that backscattering may be reduced.

The absolute value of the difference between the average refractiveindex n_(A)of the translucent resin and the second refractive indexn_(B2) of the very short fibers in the second refractive index region,|n_(A)−n_(B2) is preferably 0.01 to 0.15, more preferably 0.02 to 0.10.This makes it possible to obtain emitting light having wide diffusionproperties and inhibit the backscattering at the same time.

[Translucent Resin Film]

The translucent resin film to be used in the present invention is a filmobtained by molding a translucent rein into a film. In the translucentresin film, a plurality of very short fibers are dispersed. Thetransmittance of the translucent resin at a wavelength of 546 nm ispreferably 50% or higher, more preferably 70% or higher.

The translucent resin to be used in the present invention can be made ofany material excellent in transparency as long as a plurality of veryshort fibers can be immobilized therein in a dispersed state. Examplesof such a material for forming a translucent resin include UV-curableresins, cellulose-based polymers, and norbornene-based polymers. Thetranslucent resin is preferably made of an energy-ray curable resin,more preferably of a UV-curable resin. An energy-ray curable resin,especially a UV-curable resin can be rapidly molded into a film, whichcontributes to productivity growth.

The average refractive index n_(A) of the translucent resin ispreferably 1.3 to 1.7, more preferably 1.4 to 1.6. If necessary, theaverage refractive index n_(A) of the translucent resin can beappropriately adjusted in the same manner as the aforementionedadjusting method of the refractive index of the very short fibers.

The translucent resin to be used in the present invention is preferablyan optically-isotropic resin hardly having refractive index anisotropy.In the present invention, the word “optically-isotropic resin” refers toa resin whose birefringence (i.e., the difference between anextraordinary refractive index and an ordinary refractive index) is lessthan 0.001.

It is preferred that the translucent resin is completely embedded in thelight diffusing material such as very short fibers. However, some of thelight diffusing material may be exposed due to incomplete embedding aslong as they are immobilized.

The translucent resin film may contain any additive. Examples of such anadditive include surfactants, cross-linking agents, antioxidants, andantistatic agents. The amount of the additive contained in thetranslucent resin film is not particularly limited, but is usually 5 wt% or less with respect to the total weight of the light diffusing film.

[Production Process of the Present Invention]

A process for producing a light diffusing film according to the presentinvention comprises the steps of: A) dispersing a plurality of veryshort fibers obtained by cutting fibers in a liquid material, from whicha film made of a translucent resin can be formed, to obtain a dispersionliquid; and B) casting the dispersion liquid obtained in the step A in afilm to form a cast layer and then solidifying or curing the cast layerto obtain a light diffusing film. If necessary, the process forproducing a light diffusing film according to the present invention mayfurther comprise another step in addition to the steps A and B.

[Step A]

The step A is a step of dispersing a plurality of very short fibersobtained by cutting fibers in a liquid material, from which a film madeof a translucent resin can be formed, to obtain a dispersion liquid.

The unstretched fiber can be produced by extruding a melted polymer froma spinning nozzle. A fiber having two or more types of birefringentregions can be produced by extruding, for example, two different meltedpolymer materials from a nozzle for sea-island composite fiber spinning.Alternatively, a fiber having two or more types of birefringent regionsmay be produced by coating the surface of a single-structure fiber withanother material.

A method for obtaining very short fibers by cutting fibers is notparticularly limited. For example, a fiber bundle obtained by arranginga plurality of fibers in parallel with each other may be cut by acutting blade.

Alternatively, a method described in Japanese Patent ApplicationLaid-Open Publication No. JP 2005-113291 A may be employed. Morespecifically, a fiber bundle is impregnated with a liquid or gaseousembedding material, and then the embedding material is solidified bydecreasing the temperature to integrate the fiber bundle with theembedding material to form a single unit, and then the end face of thesingle unit is cutting-worked at a low temperature, and then theembedding material is removed by increasing the temperature to obtainvery short fibers having a length of about 0.005 mm to 1 mm.

Alternatively, a method described in Japanese Patent ApplicationLaid-open Publication No. JP 2005-126854 A may be employed. Morespecifically, a fiber bundle is impregnated with a liquid or gaseousembedding material, and then the embedding material is solidified bydecreasing the temperature to integrate the fiber bundle with theembedding material to form a single unit, and then the end faces of thethus prepared two or more single units are planed at a low temperature,and then the embedding material is removed by increasing the temperatureto obtain very short fibers having a length of about 0.005 mm to 1 mm.

Alternatively, a method described in Japanese Patent ApplicationLaid-Open Publication No. JP 2005-139573 A may be employed. Morespecifically, a plurality of fiber bundles arranged so as not to comeinto contact with each other are impregnated with a liquid or gaseousembedding material, and then the embedding material is solidified bydecreasing the temperature to integrate the fiber bundles with theembedding material to form a single unit, and then the end face of thesingle unit is cutting-worked at a low temperature, and then theembedding material is removed by increasing the temperature to obtainvery short fibers having a length of about 0.005 mm to 1 mm.

The liquid material for forming a translucent resin film is notparticularly limited. For example, a solution obtained by dissolving atranslucent resin in a solvent or a solvent-free or solvent-containingenergy-ray curable resin liquid is used.

A method for preparing the dispersion liquid is not particularlylimited. For example, the dispersion liquid may be prepared by addingthe above-described liquid material to the very short fibers placed in acontainer little by little under stirring or by adding the very shortfibers to the above-described liquid material placed in a containerlittle by little under stirring.

[Step B]

The step B is a step of casting the dispersion liquid in a film to forma cast layer and then solidifying or curing the cast layer to obtain alight diffusing film.

A method for casting the dispersion liquid in a film is not particularlylimited, and a coating method using any coater may be employed. Examplesof a coater used in a coating method include a slot orifice coater, adie coater, a bar coater, and a curtain coater.

In the step B, the cast layer is solidified or cured by any method. Inthe present invention, the word “solidified” means that a softened ormelted resin (polymer) is solidified by cooling or a resin (polymer)dissolved in a solvent is solidified by removing the solvent, and theword “cured” means that a resin (polymer) is cross-linked by exposure toheat, catalyst, light, or radiation and therefore becomes hardly solubleor meltable. The conditions for solidifying or curing are appropriatelydetermined depending on the kind of translucent resin used. In a casewhere a UV-curable resin is used as the translucent resin, theconditions for curing the UV-curable resin are to expose it to UV lightat an illuminance of preferably 5 mW/cm² to 1,000 mW/cm² so that theintegral amount of light becomes preferably 100 mJ/cm² to 5,000 mJ/cm².

[Usage of Light Diffusing Film]

The light diffusing film according to the present invention is suitablefor use in liquid-crystal panels for, for example, computers, copiers,mobile phones, watches, digital cameras, portable information terminals,portable game machines, video cameras, TV sets, microwave ovens, carnavigation systems, car audio systems, monitors for stores, surveillancemonitors, and medical monitors.

EXAMPLES Example 1

An ethylene vinyl alcohol copolymer (produced by Nippon SyntheticChemical Industry Co., Ltd. Product Name: “Soarnol DC321B,” meltingpoint: 181° C.) was fused at 270° C. and then was charged into a nozzlefor single-structure fiber spinning to obtain a spinning filament with adiameter of 30 μm by spinning the copolymer at a spinning rate of 600m/minute. This spinning filament was stretched 4 times as long as theoriginal length in warm water at 60° C. to obtain fibers with a diameterof 15 μm.

The aforementioned long fibers are aligned to form a fiber bundle andthen the fiber bundle was cut by a machining blade by fixing a polyvinylalcohol resin to be embedded therein. Subsequently, a polyvinyl alcoholresin was dissolved in hot water to be removed to obtain theaforementioned very short fibers with a fiber length of 30 μm.

A number of the above-mentioned fibers were prepared. And then thefibers were dispersed into a polyester acrylate-base ultraviolet curableresin liquid (produced by Sartomer Company Inc., Product Name: “CN2273)to prepare a dispersion liquid. This dispersion liquid was cast byflowing on the surface of a polyethylene terephthalate film to form acast layer. Subsequently, the cast layer was cured by irradiatingultraviolet rays (illuminance=40 mW/cm², amount of integrating light:1,000 mJ/cm²) and then the polyethylene terephthalate film was peeledoff to prepare a light diffusing film with a thickness of 150 μm. Themixed quantity of the very short fibers was 30 weight parts with respectto the total amount of light diffusing film. The average refractiveindex of each component and diffusing characteristics of the lightdiffusing film prepared in such a manner were as shown in Table 1.

Example 2

An ethylene vinyl alcohol copolymer (produced by Nippon SyntheticChemical Industry Co., Ltd. Product Name: “Soarnol DC321B,” meltingpoint: 181° C.) and an ethylene propylene copolymer of excessivepropylene (produced by Japan Polypropylene Corporation, Product Name“OX1066A”, melting point: 138° C.) were respectively fused at 270° C.and 230° C. and then were charged into a nozzle for sea-island compositefiber spinning (island number per fiber cross section: 37) to obtain aspinning filament with a diameter of 30 μm by spinning these copolymersat a spinning rate of 600 m/minute.

This spinning filament was stretched 4 times as long as the originallength in warm water at 60° C. to obtain fibers with a diameter of 15μm. When the cross section surfaces of the fibers were observed with anelectron microscope, it was confirmed that a sea-island structure wasconfigured wherein a columnar (diameter of its cross section:approximately 1 μm) second refractive index region (island portion)composed of an ethylene vinyl alcohol copolymer was distributed inside acolumnar (diameter of its cross section: 15 μm) first refractive indexregion (sea portion) composed of an ethylene propylene copolymer.

With the use of these long fibers, a light diffusing film with athickness of 150 μm was prepared in the same manner as in Example 1. Theaverage refractive index of each component and the diffusingcharacteristics of the thus prepared light diffusing film were as shownin Table 1.

TABLE 1 Average Average Refractive Refractive Index n_(A) of Index n_(B)of Light Diffusing Film Translucent Resin very short fibers HazeBackscattering Example 1 1.48 1.54 80% Large Example 2 1.48 Sea 80%Small portion = 1.50 Island portion = 1.54 Sea portion = Firstrefractive index region Island portion = Second refractive index region

[Assessment]

Comparing the light diffusing film (Example 1) whose very short fibersare single structured to the sea-island structured light diffusing film(Example 2), the film of Example 2 is more superior as a light diffusingfilm because the haze of both films is equivalent, however, thesea-island structured film has less backscattering. In Example 2, theaverage refractive index (1.50) of the sea portion of the very shortfibers is a value intermediate between the average refractive index(1.54) of the island portion and the average refractive index (1.48) ofthe translucent resin, so that backscattering becomes smaller.

[Measuring Method] [Haze]

Haze was measured using a haze meter (produced by MURAKAMI COLORRESEARCH LABORATORY, product name: “HM-150” in accordance with JIS K7136:2000.

[Average Refractive Index of Fibers]

A refractive index at room temperature (25° C.) and at the wavelengthsof 546 nm was measured by the Becke's line method using a polarizationmicroscope produced by Olympus Corporation.

[Refractive Index of Translucent Resin]

A refractive index at room temperature (25° C.) and at the wavelengthsof 546 nm was measured using a prism coupler produced by SaironTechnology Ltd.

[Backscattering]

A black acrylic board was adhered to the back of a light diffusing filmand a surface of the light diffusing film was illuminated by a whitefluorescent lamp to visually observe the intensity of reflected light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (a) and FIG. 1 (b) are respectively a schematic view of a lightdiffusing film of the present invention.

FIG. 2 (a) and FIG. 2 (b) are respectively a schematic view of veryshort fibers to be used in the present invention.

There have thus been shown and described a novel light diffusing filmand a process for producing the light diffusing film, which fulfill allthe objects and advantages sought therefor. Many changes, modifications,variations, combinations and other uses and applications of the subjectinvention will, however, become apparent to those skilled in the artafter considering this specification and the accompanying drawings whichdisclose the preferred embodiments thereof. All such changes,modifications, variations and other uses and applications which do notdepart from the spirit or scope of the invention are deemed to becovered by the invention, which is to be limited only by the claimswhich follow.

1. A light diffusing film comprising: a film made of a translucentresin; and a plurality of very short fibers dispersed in the film madeof a translucent resin, wherein an average refractive index n_(A) of thetranslucent resin is different from an average refractive index n_(B) ofthe very short fibers when the average refractive index n_(A) of thetranslucent resin is defined as (extraordinary refractiveindex+2×ordinary refractive index)/3 and the average refractive indexn_(B) of the very short fibers is defined as (refractive index in thedirection of a major axis+2×refractive index in the direction of a minoraxis)/3.
 2. The film according to claim 1, wherein the averagerefractive index n_(A) of the translucent resin is 1.3 to 1.7 and theaverage refractive index n_(B) of the very short fibers is 1.4 to 1.6,and an absolute value of the difference between the average refractiveindex n_(A) of the translucent resin and the average refractive indexn_(B) of the very short fibers, |n_(A)−n_(B)| is 0.005 to 0.15.
 3. Alight diffusing film comprising: a film made of a translucent resin; anda plurality of very short fibers dispersed in the film made of atranslucent resin, each of which has a first refractive index region anda second refractive index region provided within the first refractiveindex region, wherein an average refractive index n_(A) of thetranslucent resin is different from an average refractive index n_(B2)of the very short fibers in the second refractive index region when theaverage refractive index n_(B2) of the very short fibers in the secondrefractive index region is defined as (refractive index in the directionof a major axis+2×refractive index in the direction of a minor axis)/3.4. The film according to claim 3, wherein the average refractive indexn_(A) of the translucent resin is 1.3 to 1.7 and an absolute value ofthe difference between the average refractive index n_(A) of thetransparent resin and the average refractive index n_(B2) of the veryshort fibers in the second refractive index region, |n_(A)−n_(B2)| is0.01 to 0.15.
 5. The film according to claim 3, wherein the averagerefractive index n_(A) of the translucent resin, an average refractiveindex n_(B1) of the very short fibers in the first refractive indexregion, and the average refractive index n_(B2) of the very short fibersin the second refractive index region satisfies the relationship:n_(A)<n_(B1)<n_(B2) or n_(B2)<n_(B1)<n_(A) when the average refractiveindex n_(B1) of the very short fibers in the first refractive indexregion is defined as (refractive index in the direction of a majoraxis+2×refractive index in the direction of a minor axis)/3.
 6. The filmaccording to claim 1, comprising: a film made of a translucent resin; aplurality of very short fibers dispersed in the film made of atranslucent resin; and a plurality of spherical fine particles dispersedin the film made of a translucent resin, wherein an average refractiveindex of the translucent resin is different from an average refractiveindex of the very short fibers and a refractive index of the sphericalfine particles.
 7. (canceled)
 8. The film according to claim 2,comprising: a film made of a translucent resin; a plurality of veryshort fibers dispersed in the film made of a translucent resin; and aplurality of spherical fine particles dispersed in the film made of atranslucent resin, wherein an average refractive index of thetranslucent resin is different from an average refractive index of thevery short fibers and a refractive index of the spherical fineparticles.
 9. A process for producing the light diffusing film accordingto any one of claims 1 to 6 and 8, comprising the steps of: A)dispersing a plurality of very short fibers obtained by cutting fibersin a liquid material which forms a film made of a translucent resin toobtain a dispersion liquid; and B) casting the dispersion liquidobtained in the step A in a film and solidifying or curing a cast layerthereof to obtain a light diffusing film.